1. Field of the Invention
The invention relates to leakage current protection devices such as ground fault circuit interrupters. More particularly, a leakage protection device is provided that can operate using one transformer, with one magnetic core, to sense continuously both for current imbalances and low impedance ground faults.
2. Prior Art
Leakage current protection devices protect persons from electric shock hazards by sensing when current conditions on a load circuit depart from nominal. When the conditions depart sufficiently, the protective circuit decouples the load circuit from the power mains, normally requiring a reset to return to a normal state. Leakage current protection devices may be provided in domestic power outlets, such as the type of device known as a ground fault circuit interrupter (sometimes abbreviated “GFCI”), or in an appliance or in powered equipment generally.
Requirements for GFCI circuits are defined in UL standard 943A. A GFCI typically comprises a first differential transformer and a second grounded-neutral transformer. There are sensed conditions that each employ a separate transformer and magnetic core. Two primary windings of the two transformers are defined by the line and neutral power line conductors. Secondary windings on the two cores are coupled to an amplifier and switching circuit.
The differential transformer can be made of ring-shaped laminations, each lamination is stamped from a Ni—Fe sheet of 0.006 to 0.014 inches thickness. The laminations are stacked and enclosed in an electrically non-conductive encasement. The stack of laminations is termed a magnetic core or a ring core.
The power line and neutral conductor paths pass through the lumen of the ring core. The differential transformer ring core is wound with a conductor from several hundred to several thousand turns, the ring core with these windings is mounted in a plastic bracket with pins for terminations, and mounted on a circuit board.
In nominal current conditions, the line and neutral conductors carry equal and opposite currents. The differential transformer responds to an imbalance in current on the line and neutral power lines coupled through the core. A trigger signal is developed and coupled to an operator that latches open a set of contacts and thereby decouples the load circuit, including the GFCI, from the power mains.
The grounded neutral transformer in the device typically comprises a ferrite ring core. The ferrite core is wound with a secondary winding of approximately 150 to 300 turns. The grounded neutral transformer can be mounted separately or in a pair with the differential transformer in plastic brackets carrying pins for terminations.
Grounded-neutral ferrite core transformers use a dormant oscillator approach. The neutral conductor passes through the ferrite core. When resistance between neutral and ground becomes less than 2 ohms at the load side, oscillation at the ground-to-neutral transformer induces a voltage in the power line hot line and neutral conductors. Current then flows between the neutral and ground. That current represents a current imbalance and is detected by the differential core. This triggers the contacts to open and decouples the load circuit from the power mains.
Core materials such as ferrite and Ni—Fe alloys are conventionally chosen for inductor and transformer cores due to their good magnetic permeability coupled with relatively limited electrical conductivity. If electrical conductivity is high, changing magnetic flux leads to eddy currents that dissipate power due to resistive heating, which is undesirable.
A typical circuit for such a single phase GFCI as described is illustrated in FIG. 1, labeled “prior art,” and has two transformers, i.e., two functionally and structurally distinct magnetic cores. Referring to the drawing, line conductor 90 and neutral conductor 92 are coupled at the POWER SOURCE to a potential difference couple load current to and from the LOAD. Unless there is a circuit anomaly, the line and neutral currents are equal and opposite in conductors 90, 92, and are isolated from ground by a high impedance. The line and neutral conductors pass through differential transformer 111 and grounded neutral transformer 113 from the POWER SOURCE connections, for example a 120 volts ac line to the load.
A current from unequal line and neutral current levels induces a current in the secondary winding of the differential transformer 111. A voltage is thereby coupled to the input of amplifier 115. The input voltage is amplified and coupled to a contact control 117, which can include a threshold detector coupled to drive a solenoid (not shown) to open the contacts.
Differential transformer 111 typically comprises a high permeability ring core 111a and a secondary winding 111b of many turns, typically over a thousand turns. Assuming one turn for the passage of the line or neutral conductor, the transformer turns ratio is 1:1000 (or greater), to develop a current imbalance voltage of a reasonable magnitude for application to an amplifier and/or threshold detector. The grounded neutral transformer 113 has a ferrite ring core 113a and secondary winding 113b with a turns ratio typically in the range from 1:150 to 1:300.
A current imbalance between the line and neutral conductors passing through the differential transformer produces a signal voltage across secondary winding 111b, which signal is proportional in magnitude to the magnitude of the detected current imbalance. This voltage is the input to AMPLIFIER 115, which amplifies the voltage signal and produces an “open contacts” signal to CONTACT CONTROL 117, which is typically specified to open the normally-closed line contacts 101 for threshold current imbalances of 5 mA, 10 mA, 30 mA, or greater, over a temperature range of −35 C to +80 C, depending on the specific GFCI model.
In the GFCI of FIG. 1, two current situations are sensed with two respective transformers. U.S. Pat. No. 4,150,411 discloses a single transformer GFCI, but in that device the single transformer is arranged to use the single transformer alternately to sense current imbalance and low ground-neutral impedance, one at a time. For this purpose, the single ferromagnetic transformer is connected to a switching circuit that alternates between sensing current imbalances and low impedance ground faults.
Other types of leak current protection devices are known that sense current imbalance with a differential transformer, and lack a second transformer because ground faults are not sensed. For example, an appliance leakage current interrupter (ALCI) and an equipment leakage current interrupter (ELCI), which typically are mounted in the equipment housing or cabinet, typically have a differential transformer but not a grounded neutral transformer. These devices are distinct from GFCI devices that sense for both current imbalance and ground fault conditions.